Typically, forming one-dimensional multi-layered structures in a photosensitive material, e.g. so called writing of Bragg gratings, involves an interferometer in which two coherent light beams (typically in the UV wavelength range) are being directed along separate optical paths and brought to interference substantially within the photosensitive material. Within the photosensitive material, refractive index changes are induced through the interaction between the light beams and the photosensitive material, and refractive index profiles are formed due to interference patterns, whereby grating structures are written.
The characterisation of gratings during the formation process has become a significant aspect within the photonics technology field. For example, in one implementation of writing of long grating structures into an optical fibre, the quality of the written grating depends on the accuracy of matching of the velocity of an interference pattern change (sometimes referred to as travelling interference pattern) generated in the grating writing setup to the velocity of the optical fibre being translated through the interference region. In one such grating writing setup, the control of the interference pattern velocity is achieved by modulating the optical phase difference between the interfering beams e.g. by using optical modulators. Electronic control of the optical phase difference and single frequency operation of the UV laser result in an extreme accuracy, typically of the order of 10−10 that can be achieved by setting the velocity of the interference pattern using state of the art electronic equipment and stabilising the UV laser.
Unfortunately, the accuracy of the fibre motion depends on the operation of mechanically inertial translation stages and is typically not exceeding 10−3–10−5 for the translation velocity. Therefore, accurate passive synchronisation required may not be achievable with the translation stages currently available on the market.
Thus, an active feedback approach has been suggested to compensate for the inaccuracies in the fibre motion control to achieve high fidelity of such grating fabrication methods. Measuring characteristic parameters of a grating under fabrication, comparing the measured parameters to specified desired parameters, generating corrections to the grating design parameters and closing a feedback loop by applying the corrections to a grating writing control system in the process of grating writing is the underlying concept of relevant prior art. Grating design serves as a reference in this approach, with the corrections accounting for random or systematic imperfections in the grating fabrication process such as the above mentioned translation velocity inaccuracies. Therefore, the grating quality will ultimately depend on the quality of the grating measurements and the quality of the feedback loop rather than on the quality of the motion control.
A fibre Bragg grating can be fully described or characterised by either its coupling coefficient, or its impulse response, or its reflection coefficient. Those are sometimes referred to as grating design, temporal response and spectral response respectively and are related through various transforms, e.g. impulse response is Fourier transform of the complex reflection coefficient, and the coupling coefficient can be deduced from the impulse response using inverse scattering methods. Experimentally, both amplitudes and phases of the coupling and reflection coefficients as well as the ones of the impulse response are typically measured. Mathematically, the characteristic parameters with both amplitude and phase can be expressed as complex functions and, to emphasize that, we will refer to the measurands throughout the present description as to complex coupling and reflection coefficients, complex impulse response function correspondingly.
Known techniques for fibre Bragg grating characterisation can be categorised in terms of the prime measurands mentioned above or in terms of the principle of operation. Naturally, both are often related. The side diffraction techniques enable the direct measurements of the grating coupling coefficient and are based on the external Bragg diffraction at the in-fibre Bragg grating. Optical low-coherence reflectometry (OLCR) methods have been shown to produce the impulse response of the grating device under investigation. The optical frequency domain characterisation (OFDC) methods are related to coherent interferometry and provide spectral data for further analysis. The methods of so called optical space domain reflectometry (OSDR) introduce a small phase perturbation in the grating structure and the spatial variation of its parameters is derived from the grating response to the perturbation. An “industry standard” modulation phase-shift (MPS) technique is based on measuring relative phase delay between the carrier and modulation sidebands in the spectrum of the intensity modulated tunable laser and as such provides characterisation data in the spectral domain.
Overall, a general disadvantage of most of the above mentioned prior art techniques is an excessive amount of information being acquired which makes them relatively slow. This is because the measurements need to be conducted across a relevant wavelength range, and thus involve measurements of spectra and the associated necessary amount of data, and tuning of the laser source. Therefore, the grating fabrication speed may need to be slowed down to account for e.g. a slow scanning speed of a frequency-swept laser used in the OFDC approach. However, it has been recognised by the applicant that the excessive data collection may not be needed at all, should a different approach to acquiring characterisation data be taken.
The present invention seeks to provide an alternative grating characterisation method which can provide the basis for real-time grating characterisation suitable for writing of gratings with the active feedback approach.